Branching units of optical fiber transmission systems, particularly sea branching units of submarine fiber transmission systems, use a feed line, which has three connection terminals A, B and C, and can connect the power feeding lines between trunks or between a trunk and a branch as desired by connecting two terminals. Since power can be supplied to other power feeding lines connected to the last terminal independently from power feeding lines connected to the former two of terminals by connecting the last terminal to ground, the circuit typically has a circuit arrangement that can selectively connect terminals B and C to ground.
FIGS. 4A through 4D show aspects of connection of a conventional feed line connecting circuit. FIG. 4A shows an upowered condition, FIG. 4B shows the condition where the terminal A and the terminal B are connected, FIG. 4C shows the condition where the terminal A and the terminal C are connected, and FIG. 4D shows the condition where the terminals B and C are connected to ground. In FIGS. 4B through 4D, signs (+) and (-) show relative potentials. By establishing any of these potential conditions, a desired configuration of connection is selected.
Under the unpowered condition shown in FIG. 4A, terminals A, B and C are usually connected to each other and float from ground to permit an insulation electric test. Due to a current flowing from the terminal A to the terminal B, which applies a plus potential to the terminal A and a minus potential to the terminal B, the terminal C is disconnected from the terminals A and B and connected to ground as shown in FIG. 4B, and this state of connection is maintained. When a plus potential is applied to the terminal A while a minus potential is applied to the terminal C, the terminal B is disconnected from the terminals A and C and connected to ground as shown in FIG. 4C, and this state of connection is maintained. When a minus potential is applied to both terminals B and C, all terminals A, B and C result in being connected to ground as shown in FIG. 4D.
As shown in FIGS. 4B and 4C, the conventional feed line connecting circuit is configured to always connect one or more of out-of-use feed line connecting terminals to ground.
In recent submarine optical fiber transmission systems in which optical fiber cables are connected in form of a ring, a double branching system connecting two branching units to each branch station has been proposed to increase the reliability of the branch station. FIG. 5 is a block diagram schematically showing the construction of a double branching type of power feeding system. Reference numerals 10 and 12 denote trunk stations, 14 is a branch station, and 16a and 16b are (feed line connecting circuits of) branching units. A power feeding line 20 connects the trunk station 10 to the feed line connecting terminal A of the branching unit 16a, and a power feeding line 22 connects the feed line connecting terminal C of the branching unit 16a to the branch station 14. The power feeding line 24 connects the branch station 14 to the feed line connecting terminal C of the branching unit 16b, and the power feeding line 26 connects the feed line connecting terminal B of the branching unit 16b to the trunk station 12. A power feeding line 28 connects the feed line connecting terminal B of the branching unit 16a to the feed line connecting terminal A of the branching unit 16b.
Since the branching units 16a, 16b are installed in relatively short distances around 20 km, the trunk cables used there are usually configured for non-repeated transmission, and need no power source. In FIG. 5, the feed line connecting circuit of the branching unit 16a is in the condition of FIG. 4B where the feed line connecting terminals A and C are connected, rendering the trunk station 10 in a plus potential and the branch station 14 in a minus potential, and power is supplied to the power feeding lines 20 and 22 between the trunk station 10 and the branch station 14. The feed line connecting circuit of the branching unit 16b is in the state of FIG. 4D where all feed line connecting terminals A, B and C are connected to ground, rendering both the branch station 14 and the trunk station 12 in a minus potential, and the power feeding lines 20, 22 are supplied with power independently. The power feeding line 28 are connected to sea grounds at opposite ends thereof.
When the power feeding line 28 is grounded at both ends thereof to sea grounds, a loop current flows due to a difference in potential between the grounds, and the current invites corrosion of the anode side ground electrode.
Moreover, when the power feeding line 22 breaks and becomes electrically open, the feed line connecting circuit of the branching unit 16a changes to the configuration shown in FIG. 4B where the feed line connecting terminals A and B are connected to each other, and the power feeding system exhibits the configuration shown in FIG. 6. That is, the power feeding line 20 connects to the power feeding line 28, and falls to ground at the feed line connecting terminal A of the branching unit 16b. In the feed line connecting circuit of the branching unit 16a, a large current flows when connection of the feed line connecting terminal B changes from ground to the feed line connecting terminal A, and may damage the relay circuit in which the current flows.